Smoke detector with means for compensating for variations in light source brightness due to line voltage variations

ABSTRACT

A smoke detector having voltage control circuit for compensating for changes in light brightness caused by variations in line voltage. The voltage control circuit comprises a voltage regulating circuit that provides a higher voltage across the photo-cell detector when the line voltage is lower than normal, and vice versa.

Unlted States Patent [1 1 [111 3,789,383

Vasel Jan. 29, 1974 [54] SMOKE DETECTOR WITH MEANS FOR 3,314,058 4/1967 Osborne 340/237 S UX COMPENSATING FOR VARIATIONS IN 2,069,160 1/1937 Grant, Jr. 340/237 S 2,415,179 2/1947 Hurley, Jr. 340/228 5 ux LIGHT SOURCE BRIGI-ITNESS DUE TO LINE VOLTAGE VARIATIONS Alfred W. Vasel, Abington, Mass.

Pyrotector Incorporated, Hingham, M ass.

Filed: Dec. 13, 1971 Appl. No.: 207,461

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 9/1968 Maynard 323/36 UX 9/1969 Fricker 323/36 UX OTHER PUBLICATIONS RCA Technical Notes No. 741; Combined Smoke and Fire Detection Circuit for the Home by E. Fischer; 2 sheets; January, 1968.

Primary Examiner-John W. Caldwell Assistant Examiner-Daniel Myer Attorney, A gent, or Firm Robert E. Ross [57] ABSTRACT 5 Claims, 9 Drawing Figures SMOKE DETECTOR WITH MEANS FOR COMPENSATING FOR VARIATIONS IN LIGHT SOURCE BRIGIITNESS DUE TO LINE VOLTAGE VARIATIONS BACKGROUND OF INVENTION One type of smoke detector in wide use utilizes a dark chamber through which ambient atmosphere is forced or allowed to diffuse, with means projecting a light beam across the interior of chamber. A photo-cell is positioned to view the medial portion of the light beams, and is shielded from the direct radiation of the light source.

When the atmosphere entering the chamber contains smoke particles, light reflected therefrom onto the photocell causes a decrease in resistance of the photo-cell. When the resistance of the cell falls to a predetermined value, corresponding to a predetermined smoke concentration, the drop in resistance of the cell is utilized to activate an alarm circuit.

One of the undesirable characteristics of devices of this type is the fact that the concentration of smoke at which the device goes to the alarm condition varies with variations in line voltage. This is due principally to the fact that the amount of light reflected onto the photo-cell from smoke particles is a function of the brightness of the light beam, which is dependent on the voltage applied to the light source. With a line voltage higher than normal, the light source will be brighter than normal, and hence the alarm not only will sound at a concentration lower than the predetermined amount, but also will be susceptable to false alarms. With a line voltage lower than normal, the light source will be less bright than normal, a greater smoke concentration will be required to activate the alarm circuit, which could greatly delay the response time of the unit.

Although various means have been used to regulate the voltage applied to the light source, the additional circuitry required adds to the cost of the unit, and does not compensate for the variation in voltage across the plate-cell, which variation also causes a variation in the amount of smoke concentration necessary to activate the alarm.

SUMMARY OF THE INVENTION In accordance with this invention, a smoke detector is provided in which voltage means is provided for varying the voltage applied to the photo-cell inversely with variations in line voltage, to compensate both for changes in brightness of the light source due to the variation in voltage applied thereto, and to eliminate the changes in voltage that would be applied to the photocell in the absence of such voltage varying meansflnexpensive circuit means is provided for increasing the voltage at the photo-cell when the line voltage drops, to make the unit more sensitive and thereby to compensate for the decreased intensity of the light source; and for decreasing the voltage at the photo-cell when the line voltage rises, to make the unit less sensitive, to compensate for the increased brightness of the light source.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a top plan view, partly broken away, of a smoke detector labyrinth for use with the circuit described herein.

FIG. 2 is a view in section taken in line 33 of FIG.

FIG. 3 is a schematic diagram of a smoke detector electrical circuit embodying the features of the invention.

FIGS. 4, 5 and 6 are diagrams of the wave form of the applied voltage at the input to the neon voltage regulating bulb at low line voltage, normal line voltage, and high line voltage respectively.

FIGS. 7, 8, and 9 are diagrams of the wave form of the applied voltage at the input to the detector cell at low line voltage, normal line voltage and high line voltage respectively.

Referring to the drawing, there is illustrated a smoke detector, which includes a housing 12, a light source 14 disposed outside the housing, and a detector element Cl positioned to view the interior of the housing in a manner to appear hereinafter.

The housing 12 comprises a peripheral wall 18 having a pair of end caps 20 and 22 forming an internal dark chamber 24. The end caps extend beyond the periphery of the wall 18 and have inwardly turned flanges 26 and 28 which are spaced outwardly from the wall. Each end of the wall is provided with a series of outwardly inclined spacing lugs 30 which are adapted to engage frictionally the inner surface of the flanges to retain the caps in assembly. The flanges 26 and 28, in conjunction with the spacing lugs 30, form a peripheral opening at each end of the wall to permit smoke to enter the chamber from the surrounding atmosphere.

To direct and control the beam from the light source 14, a focusing tube 32 extends through the housing wall on one side thereof, and a light trap tube 34 is disposed in the housing wall on the opposite sign thereof in alignment with the focusing tube 32.

A lens 36 may be positioned in the focusing tube to focus the light beam into the light trap.

A detector tube 38 extends through the housing wall between the light trap tube and the focusing tube, and is generally perpendicular to the axes thereof.

The detector element C1 is mounted in the detector tube with a lens 40 positioned in front of the detector to restrict the field of view of the cell to the medial portion of the light beam.

The detector element Cl may be any type of device which is responsive to an increase in light intensity in a reduction in resistance, such as a cadmium sulfide photo-resistive cell.

Hence in the illustrated embodiment, a detector circuit may be connected to the detector element Cl and adjusted to the resistance thereof under normal conditions of no smoke, so that in a predetermined further decrease in cell resistance due to reflected light from smoke in the housing, an alarm will be actuated.

The physical structure and operation of the device is similar to that described in U.S. Pat. No. 3,382,762 issued May 14, 1968.

Rererring to FIG. 3, there is illustrated an alarm control circuit for use with the labyrinth of FIGS. 1 and 2.

The circuit of FIG. 3 is energized from a 115 VAC source E and is intended to complete a circuit to an alarm device A when a predetermined amount of smoke appears in the housing. For this purpose the cell C1 is connected across a voltage control circuit VC (connected within the dotted line) in series with a resister R1. The junction J1 between the resister RI and the cell C1 is connected to a neon bulb N1 which is connected to the gate of a silicon controller rectifier SCR. The anode-cathode path of SCR is connected in series with the alarm device A.

When the resistance of the cell C1 drops to a predetermined value due to reflected light from smoke in the housing, the voltage at J1 rises to a value high enough to allow the neon bulb N1 to conduct; allowing the voltage at junction J2, between the SCR gate and resistor R2 to rise to a point that the SCR is triggered into conduction in its anode-cathode path to energize the alarm A. During the increase in voltage at junction JR, the charge in capacitor Fl increases, so that when the neon tube conducts, sufficient energy is present to give an adequate pulse of current through the neon bulb N] to raise the voltage at the SCR gate to the desired level.

As previously described, at any given smoke concentration in the housing, the resistance of the photo cell Cl will vary with the brightness of the light and therefore, unless some compensation is provided, the smoke concentration at which the alarm will be energized will vary with the supply voltage.

For example, it has been found that commercial detectors with no provision for compensation for light brightness, if set to energize the alarm at 3 percent smoke when the line voltage is 115 volts, will require a concentration of over 6 percent smoke when the line voltage is 100 volts, and will energize the alarm at less than 1 percent smoke when the line voltage rises to 132 volts. (The term percent smoke, as used herein refers to the drop in light intensity when passing through a chamber of known length. For example, if the light intensity is reduced 3 percent by passing through a column of smoke 2 feet long, it is considered that the column contains 3 percent smoke.)

In the illustrated embodiment of the invention, the voltage control portion of the circuit, VC, is provided. This portion of the circuit provides an increased voltage across the photo-cell when the line voltage is higher than normal, in a manner now to be described.

The portion VC of the circuit comprises a neon bulb N2 connected across the voltage source, and a resistor R3, a diode D1 and a capacitor F2 connected in series across the voltage source. The junction J3 between the diode D1 and the capacitor F2 is connected to the photo-cell C1 to serve as the voltage supply for said cell.

This portion of the circuit operates in the following manner. Since the supply voltage is 60 cycle alternating current, the voltage at the resistor R3 during a half cycle, (in the absence of the neon bulb N2) would rise from zero to the peak voltage and fall again to zero at the end of the half cycle. The capacitor F2, which receives only half-wave current through the diode D1, would then aquire a charge which would be a function of the peak voltage reached during each half-cycle. However, the presence of the neon bulb N2, which has a breakdown voltage ofjust below the normal peak line voltage of 150 volts, regulates the voltage appearing at the input of resistor R3 by not conducting until its breakdown voltage of about 150 volts is reached, and then maintaining conduction at a lower voltage level. As shown in FIG. 4, it is seen that at a supply voltage lower than normal, such as volts RMS, the peak voltage is about 141 volts, which is less then the neon breakdown voltage, and the wave form at the input to the resistor R3 has substantially the shape of the sine wave.

When the line voltage is normal, or about volts, the peak voltage is about I62 volts, or just above the firing voltage of the neon. As illustrated in FIG. 5, it is seen that when the supply voltage reaches the breakdown voltage of the neon, the neon conducts and the voltage across the neon drops to a lower level of approximately 100 volts, due to the voltage drop across the resistor R4 disposed between the neon bulb N2 and the voltage source. A spike of voltage S N is thereby produced during the beginning half of each cycle in the wave form appearing at the input of resistor R3.

When the line voltage is above normal, for example volts RMS, with a peak voltage of about 198 volts the striking voltage of the neon is reached sooner on each half cycle than when the line voltage is normal, as illustrated in FIG. 6. The spike of voltage 8,, produced in each cycle is therefore sharper than the spike produced when the line voltage is normal.

The wave forms appearing at the resistor R3, illustrated inFIGS. 4, 5, and 6, are modified by the filtering effect of the RC combination of R3 and F2, so that the wave form appearing at the input of the cell Cl and capacitor F2 under conditions of low line voltage, normal line voltage, and high line voltage is shaped as illustrated in FIGS. 7, 8, and 9 respectively. The filtering action of the RC combination in the wave-form of low line voltage is very slight, causing only a slight reduction in peak voltage.

The filtering action on the wave-form of normal line voltage is considerably greater, and, by filtering out a portion of the spike S which is in effect a highfrequency pulse, the peak voltage reached is reduced to a value less than the peak voltage reached when the line voltage is below normal. (Compare FIG. 5 with FIG. 6.)

The filtering action on the wave-form of high line voltage is greater than on the wave-form of normal line voltage since the sharper spike 8,, is, in effect, a higher frequency pulse, and it is therefore smoothed at a lower voltage level. (Compare FIG.6 with FIG. 7.)

The voltage maintained on the capacitor F2 determines the current flow through the cell Cl, which determines the voltage at the junction J1 and the firing point of the trigger neon N1.

The charge on the capacitor is a function of the peak voltage appearing at the capacitor input. As shown in FIGS. 7, 8, and 9, it is seen that the peak voltage decreases as the supply voltage increases. Therefore, with a higher supply voltage, the current flow through the cell Cl at a given cell resistance will be less, giving a lower voltage input to the trigger neon N1. However, the higher supply voltage also provides more light output from bulb 14, and with a given amount of smoke in the housing more light falls on cell Cl causing a lower than normal cell resistance. The fact that said higher supply voltage also results in a lower voltage at the input of cell Cl compensates for said increase in light falling on the cells.

With the selection of components of appropriate value, the compensation afforded by the portion VC of the circuit can almost exactly compensate for the effect of variations in light output caused by variations in line voltage, so that with a given amount of smoke in the housing, the current flow through the cell, and therefore the voltage at junction J1, does not vary with variations in line voltage.

Although in the illustrated embodiment of the invention the resistor R3, diode D1 and capacitor F2 are included as part of the voltage control circuit, in some cases it is possible to omit these components and still have an operative device with voltage compensation, provided that the impedance of the cell Cl is of the proper value to provide, in combination with the capacitor Fl, the filtering effect previously described, so that changes in the supply voltage will cause opposite changes in the voltage appearing at the neon bulb N1. However, it has been found that the impedance value of photo-cells available commercially varies so widely that inclusion of the above mentioned components is necessary to obtain consistent performance.

Since certain other changes may be made in the illustrated embodiment of the invention without departing from the scope of the invention, it is intended that all matter contained herein be interpreted in an illustrative and not a limiting sense.

1. In a smoke detector of the type designed to be energized from an AC. source and having a photoresistive cell positioned to view smoke particles illuminated by the light source, the improvement comprising means for varying the effective voltage applied to the photo-resistive cell inversely with variations in voltage applied to the light source, whereby the increase or decrease in brightness of the light source with increase or decrease in supply voltage does not appreciably change the alarm point of the detector due to the compensating decrease and increase of voltage across the photocell with the increase and decrease respectively of the brightness of the light source.

2. In a smoke detector of the type designed to be energized from an AC. source and having a light source, a photocell positioned to view smoke particles illumi nated by the light source, and an alarm actuating circuit responsive to an increase in voltage resulting from a drop in resistance of the photocell to actuate an alarm, the improvement comprising means for causing the voltage at the alarm actuating circuit to decrease when the supply voltage increases and vice-versa, said means including a capacitor connected to serve as a D.C. voltage supply for the alarm actuating circuit, means for applying a D.C. charging voltage to said capacitor from the supply voltage, and means for reducing the effective D.C. voltage applied to the capacitor during each half cycle with increases in supply voltage.

3. In a smoke detector of the type adapted to be energized from an AC. source and having a light source, a photo'cell positioned to view smoke particles illuminated by the light source, and means responsive to change in D.C. voltage drop across the photo-cell to actuate an alarm, wherein a change in the supply voltage will cause a change in the brightness of the light source and a consequent change in the light received by the photo-cell at a given smoke concentration, the improvement comprising means for causing the D.C. voltage across the photo-cell to decrease when the supply voltage increases and vice-versa, said means including a capacitor connected to serve as the voltage supply for the cell, means charging said capacitor with half-wave current from the same source that supplies the light source, a neon bulb connected across said capacitor to control the effective charge applied thereto, whereby as the supply voltage increases, the neon goes into conduction at an earlier point in the charging half cycle and the high frequency voltage spike formed by the clipping action of the neon bulb filters through the capacitor more readily than the spike formed by the neon bulb at a lower line voltage, and the maximum voltage acquired by the capacitor decreases.

4. In a smoke detector of the type designed to be energized from an AC. source and having a light source, a photo-resistive cell positioned to view smoke particles illuminated by the light source, means for supplying a D.C. voltage across the cell, and means responsive to a change in drop in voltage across the cell to actuate an alarm, the improvement comprising means for compensating for a change in brightness of the light due to a change in supply voltage, said means causing a decrease in voltage applied to the photo-cell when the supply voltage to the light increases, and vice-versa.

5. A smoke detector as set out in claim 4 in which said means for supplying voltage across said cell and for compensating for a change in the brightness of the light comprises a capacitor and a diode in series across a neon voltage regulator bulb, the voltage supply to the cell being taken from the junction between the diode and the capacitor. 

1. In a smoke detector of the type designed to be energized from an A.C. source and having a photo-resistive cell positioned to view smoke particles illuminated by the light source, the improvement comprising means for varying the effective voltage applied to the photo-resistive cell inversely with variations in voltage applied to the Light source, whereby the increase or decrease in brightness of the light source with increase or decrease in supply voltage does not appreciably change the alarm point of the detector due to the compensating decrease and increase of voltage across the photo-cell with the increase and decrease respectively of the brightness of the light source.
 2. In a smoke detector of the type designed to be energized from an A.C. source and having a light source, a photocell positioned to view smoke particles illuminated by the light source, and an alarm actuating circuit responsive to an increase in voltage resulting from a drop in resistance of the photocell to actuate an alarm, the improvement comprising means for causing the voltage at the alarm actuating circuit to decrease when the supply voltage increases and vice-versa, said means including a capacitor connected to serve as a D.C. voltage supply for the alarm actuating circuit, means for applying a D.C. charging voltage to said capacitor from the supply voltage, and means for reducing the effective D.C. voltage applied to the capacitor during each half cycle with increases in supply voltage.
 3. In a smoke detector of the type adapted to be energized from an A.C. source and having a light source, a photo-cell positioned to view smoke particles illuminated by the light source, and means responsive to change in D.C. voltage drop across the photo-cell to actuate an alarm, wherein a change in the supply voltage will cause a change in the brightness of the light source and a consequent change in the light received by the photo-cell at a given smoke concentration, the improvement comprising means for causing the D.C. voltage across the photo-cell to decrease when the supply voltage increases and vice-versa, said means including a capacitor connected to serve as the voltage supply for the cell, means charging said capacitor with half-wave current from the same source that supplies the light source, a neon bulb connected across said capacitor to control the effective charge applied thereto, whereby as the supply voltage increases, the neon goes into conduction at an earlier point in the charging half cycle and the high frequency voltage spike formed by the clipping action of the neon bulb filters through the capacitor more readily than the spike formed by the neon bulb at a lower line voltage, and the maximum voltage acquired by the capacitor decreases.
 4. In a smoke detector of the type designed to be energized from an A.C. source and having a light source, a photo-resistive cell positioned to view smoke particles illuminated by the light source, means for supplying a D.C. voltage across the cell, and means responsive to a change in drop in voltage across the cell to actuate an alarm, the improvement comprising means for compensating for a change in brightness of the light due to a change in supply voltage, said means causing a decrease in voltage applied to the photo-cell when the supply voltage to the light increases, and vice-versa.
 5. A smoke detector as set out in claim 4 in which said means for supplying voltage across said cell and for compensating for a change in the brightness of the light comprises a capacitor and a diode in series across a neon voltage regulator bulb, the voltage supply to the cell being taken from the junction between the diode and the capacitor. 